Frequency-modulated oscillator



Dec. 8, 1953 E. H. HUGENHOLTZ 2,662,214

FREQUENCY-MODULATED OSCILLATOR Filed July 50, 1951 FREQUENCY MUL T/PL/ERPHASE MODULATOR PHASE-COMPARISON smea- STABLE OSCILLATOR INTEG'RA TINGNETWORK\ INVENTOR Eduard Hermon Hugenhol'rz AGEN T Patented Dec. 8, 1953UNITED FREQUENCY-MODULATED OSCILLATOR Eduard Herman Hugenholtz,Hilversum, Netherlands, assignor'to Hartford National Bank and TrustCompany,- Hartford, Conn as trustee Application July 30, 1951, SerialNo. 239,232

The invention relates to circuits for the frequency-modulation of anoscillator of the kind in which the oscillator frequency is synchronisedwith a frequency derived from a phase-modw lator stage (which stage iscontrolled by the modulating oscillation) with the aid of aphasecomparing stage whose output controls the irequency-governingelement of the oscillator. In this manner the frequency produced by theoscillator may be rendered identical with the he quency derived from thephase-modulator stage.

In the known circuits of this kind the controlvoltage responsible forthe frequency-sweep of the oscillator is entirely supplied by thephasecomparing stage. We have found that there a tendency towardsinstability in these known circuits and, more particularly, a tendencytowards self-oscillation of the closed circuit formed by the oscillator,the phase-comparing stage and the frequency-governing element.

According to the invention a material improvement is achieved as regardsstability by additionally supplying the modulating oscillation to thefrequency-governing element of the oscillator in such manner as topermit reduction of the oscillation-amplitude needed at the output ofthe phase-comparing stage for supply to the said element.

In order that the invention may be more clearly understood and readilycarried into effect, one embodiment will now be described with referenceto the accompanying drawing, in which:

Fig. 1 shows a block diagram, and

Fig. 2 is a detail view of part of the diagram shown in Fig. 1.

Referring to Fig. 1, designates a frequencymodulated oscillator and 2 aphase-comparing stage, for example a mixer, the output of whichcomprises a bandpass filter passing a band of relatively lowfrequencies. Reference 3 desighates a phase modulator, with the aid ofwhich the oscillation produced by a stable oscillator for example acrystal oscillator is phase-modulated in accordance with a modulatingoscillation supplied to the phase-modulator 3 from the terminals 5 byway of an integrating network t. The phase of the oscillation producedby the os cillator I is compared in stage 2 with the phase of anoscillation derived from the phase-modulator stage 3 via, for example,one or more frequency multiplying stages i and the output of stage 2 isfed at a to a frequency-governing element 8, for example a reactancetube or other control-tube, controlling the oscillator.

The oscillation produced by the oscillator ,i is

3 Claims. (Cl. 332-19) thus synchronised with the output oscillation ofthe frequency multiplier 7, these two oscillations having identicalfrequencies. Consequently the oscillator I produces afrequency-modulated oscillation corresponding with the modulatingoscillation across the terminals 5, the central frequency of thisoscillation being determined by the frequency of the stable oscillator3.

The circuit so far described is of the known kind already defined and,as stated, has a tendency to instability, which is due according to theinvestigations we have carried out to'the fact that the closed circuitformed by the oscillator l, the phase-comparing stage 2 and thefrequency-governing element 3 exhibits a negative feedback actionoperating against the phase divergence between the oscillation of theoscillator and the output oscillation of the frequency multiplier I; solong as the phase-comparing stage 2 has to supply (in the known manner)the full modulation voltage to control the oscillator I, this feedbackmust be comparatively strong, so that the probability ofself-oscillation of this circuit arises.

The probability of instability is materially reduced when in accordancewith the invention, the modulating oscillation is additionally suppliedat b to the frequency-governing element 8 of the oscillator I and thephase-comparing stage 2 need only supply a voltage which is proportionalto the difference between the central frequency of the outputoscillation of the frequency multiplier '1 and the central frequency ofthe oscillation which the oscillator I would produce if the controlacross the phase-comparing stage 2 and the frequency-governing element 8were not provided.

Instead of using the frequency multiplier 1. or in conjunction herewith,use may be made with advantage of a pulse stage, which converts theoutput of the phase modulator stage 3 into pulses of short duration, usethen being made of a gated mixing stage for the phase-comparing stage 2.In this case the frequency of the oscillator i may be equal to a veryhigh harmonic of the frequency of the phase-modulator stage 3.

Referring to Fig. 2 a suitable frequency-governing element 8 for use inaccordance with the invention is shown more in detail. In order toensure that the phase-comparing stage 2 need supply only direct voltageand very low frequencies it is advisable to include the parallelcombination of two control tubes in the frequencygoverning element 8.One of these tubes (8', Fig. 2) has a large control-range but is onlycontrolled 3 by the direct voltage component and the slow variations ofthe voltage formed by the combination of the output voltage (a) of thephasecomparing stage 2 and the modulating voltage (17) across theterminals 5, whereas the other control tube (8") is adjusted to aconstant grid bias voltage and hence to a constant mutual conductance;This latter" tube 8" receives from a capacitor ID the alternatingvoltage component of the output voltage (a) of the phase-comparing stage2 and through a potentiometer H the voltage (b) across the terminalsizg'withztheaid'of. this potentiometer and, if necessary, with the..use. ofadditional phase-correcting elementstitlisv en: sured that at least forthe highenmodulati'onfreequencies the phase comparing stage 2 need notsupply voltage. The total output; c urrrmt' oi the tubes 8' and 8"controls the frequency, of the oscillator I.

If necessary, additional'measures may be taken in order toensure also inthe case ofa changeover of the oscillators I and l to another frequencythat the phase-comparing stage 2'. has practically not to supplyalternating voltage.

In order to suppress any further't'endency towards instability of thecontrol-circuit due to phase shift in the output circuit of' thephasecomparing stage 'i, the output network of the phase-comparing stage2" may havec'onnected in parallel with it a tuned series circuit whichraises the critical frequency atwhich the closed circuit I, 2,. 8 mighttend to self-oscillate.

What I claim. is:

l. A circuitfor. frequency-modulating an oscillator with a modulatingvoltage andfor synchro'e nizing said oscillator. comprising a frequencygoverning device coupledto. said? oscillator for varying the frequencythereof, ,a source of stable oscillations, means including a, phasemodulator coupled to said source to produce a synchronization wavephase-modulated by said modulating voltage, a phase comparison deviceresponsive to said synchronization wave and coupled to said oscillatorto produce a control voltage, means to apply said control voltage tosaid governing device to vary the frequency of said oscillatoraccordingly, and: means t'o'eapply; saint modulating voltage to saidgoverning device also to frequencymodulate said oscillator, whereby areduction in the amplitude requirements for said control voltagezsisinfected.

2... A. circuit, as setrforth in claim 1, wherein saidl'sourcelofstablezoscillations operates at a frequency', which as sub-harmonic of the meanfrequency of said oscillator, and said means to producea synchronizationwave includes a frequency multiplier to raise the frequency produced bysaid' source to the frequency corresponding to said mean frequency.

3'; A circuit; as set forth inclaim 1, wherein said frequencygoverningdevice is' constituted by first andsecond reactance tubecircuits; coupled inparallel relation to said oscillator; said firstreactance tube circuit having a relatively large control range withrespect to the second circuit, means to apply said control voltage tothe input of. said first reactance' tube circuit, and means to applysaid modulating voltage to the input of said second. reactance' tubecircuit..

EDUARD HERMAN HUGENHOLTZ.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,279,659 Crosby Apr. 14, 1942 2,310,260 Schock Feb. 9,1943-

